Logic circuits employing Josephson junction devices can be broadly classified into circuits driven by an A.C. source and circuits driven by a D.C. source. The A.C.-driven circuit has the advantages that the circuit speed is high, that the signal voltage amplitude is large, and that the load driving capacity is large. However, in case of applying it to a large-sized computer system of high operating speed, it has caused the following problems:
(1) Since the phase of the A.C. source corresponds to the clock of the logic circuit, the phases of the A.C. sources for the respective logic circuits need to be precisely adjusted. Especially when large quantities of logic circuits are used as in the large-sized computer, it is difficult to precisely adjust the A.C. source phases of all the logic circuits.
(2) The process in which the Josephson junction device shifts from the non-superconducting state into the superconducting state, cannot be applied to a logical operation. Therefore, when a logic arrangement is constructed, the number of circuits becomes larger with the A.C.-driven circuit than in case of relying on semiconductor technology.
(3) The large-sized computer system of fast operation requires an oscillator of high power and high frequency.
(4) In order to prevent the Josephson junction device from shifting from the superconducting state into the non-superconducting state unnecessarily, noise to be involved in a signal must be made very small. Particularly in the case of the large-sized computer, it is difficult to adjust and clear all pulse waveforms.
(5) When the current source is switched fast, there occurs the so-called punch-through phenomenon in which the Josephson junction device used in the circuit fails to return into the superconducting state. This phenomenon occurs at a probability, and when a large number of circuits are used as in the large-sized computer, the probability at which the punch-through phenomenon occurs in the whole system becomes high, so that the reliability of the whole system is lowered.
(6) An A.C. distribution system assumes a large installation area, and the packaging density of the A.C.-driven circuit does not become high.
It is therefore difficult to construct the large-sized computer system out of only the A.C.-driven circuits.
The D.C.-driven circuit is free from the problems of the A.C. source drive system already mentioned. Known circuits based on prior arts, however, have caused the following problems:
(1) The D.C.-driven circuit is a circuit which cannot drive a transmission line, or even when it can drive a transmission line, it is a circuit of small signal amplitude voltage. Therefore, the load driving capacity is small.
(2) The D.C.-driven circuit is a circuit which cannot drive a transmission line, or is a circuit which can drive only a transmission line of low characteristic impedance.
This causes such problems as:
(a) inferior matching with a transmission line outside an LSI, and
(b) great delay due to an inductive load.
It is therefore difficult to construct a large-sized computer of a structure employing a large number of LSIs and connecting the LSIs by transmission lines, out of only D.C.-driven circuits.